1. Field of the Invention
The present invention generally relates to the art of reciprocating piston assemblies, and more specifically to a linear compressor including a reciprocating piston and an integrally machined double-helix piston spring which exerts substantially no lateral reaction force on the piston.
2. Description of the Related Art
A linear compressor including a double-opposed piston assembly is known in the art per se and used for generating a pulsating pressure which is applied to reciprocate a displacer piston in a Stirling cycle cooler as described in a publication entitled "ADVANCES IN CRYOGENIC ENGINEERING, Vol. 19, paper F-3 entitled "PNEUMATICALLY DRIVEN SPLIT-CYCLE CRYOGENIC REFRIGERATOR", by S. Horn et al, pp. 216-220.
The linear compressor is illustrated in FIG. 1 and designated as 10. A variable compression chamber 12 is defined within a housing 14 by cylinders 16 and 18 which are joined at their inner ends. Pistons 20 and 22 are slidable in the cylinders 16 and 18 for movement toward and away from the center of the compression chamber 12 and movably define the longitudinal ends of the compression chamber 12.
The pistons 20 and 22 are reciprocated by linear moving coil motors 26 and 28 respectively in a synchronized manner. More specifically, the motors 26 and 28 move the pistons 20 and 22 respectively toward the center of the compression chamber 12 together and away from the center of the compression 12 together in symmetrical reciprocating movements. Movement of the pistons 20 and 22 toward the center of the compression chamber 12 causes the pressure therein to increase, and vice-versa.
The compression chamber 12 is connected to a load such as a regenerator of a cryogenic cooler (not shown) through an outlet 24. The compression chamber 12, outlet 24, regenerator and interconnecting conduit are filled with a working gas such as helium such that reciprocation of the pistons 20 and 22 causes the working gas pressure to pulsate which in turn causes a piston in the regenerator to reciprocate and generate a thermodynamic Stirling cycle which causes absorption of heat from the cooled end and rejection of heat from the other end of the regenerator.
The motor 26 includes a generally cup-shaped armature 30 which is fixed to the piston 20 for integral movement. A coil 32 is wound around the armature 30 and connected to an electronic drive unit (not shown) by a lead 34. An annular permanent magnet 36 is fixed to the inner surface of the housing 14 radially outward of the coil 32. A helical spring 38 has a left end fixed to the left end of the housing 14 and a right end attached to the armature 30.
The piston 20, armature 30 and spring 36 are illustrated a central position in which the spring 38 is in a free state. An alternating current electrical drive signal is applied to the coil 32 through the lead 34 which causes the coil 32 to produce a magnetic field through the electromagnetic effect.
When the drive signal has a first polarity, the magnetic fields of the coil 32 and magnet 36 have relative polarities such that the coil 32, armature 30 and piston 20 are moved rightwardly toward the center of the compression chamber 12 to increase the pressure therein. This causes the spring 38 to extend and exert a force on the armature 30, piston 20 and coil 32 which urges them to return to the central illustrated position.
Reversal of the polarity of the drive signal causes the armature 30, piston 20 and coil 32 to move leftwardly away from the center of the compression chamber 12 to a position which is leftward of the center position to decrease the pressure in the compression chamber 12. This causes the spring 38 to compress and exert a force on the armature 30, piston 20 and coil 32 which urges them to return to the central illustrated position. The cycle is repeated periodically to produce a pulsating pressure in the compression chamber 12 and interconnected elements.
The motor 28 includes an armature 40, coil 42, lead 44, permanent magnet 46 and spring 48 which are identical to the corresponding elements of the motor 26. The illustrated configuration is known in the art as a "moving coil" design since the coils 32 and 42 move relative to the fixed magnets 36 and 46. However, equivalent results can be obtained by mounting permanent magnets on the armatures 30 and 40 and fixed coils on the housing 14 to provide a "moving magnet" design.
The springs 38 and 48 are typically formed of round spring wire stock in a single helical arrangement as illustrated in FIGS. 2 and 3. The problem with the springs 38 and 48 in a reciprocating piston assembly such as in the compressor 10 is that the springs 38 and 48 exert lateral reaction forces on the pistons 20 and 22 which cause them to wear from their original straight cylindrical shape into a barrel shape.
This causes the seals between the pistons 20 and 22 and the cylinders 16 and 18 respectively to degenerate, the pressure ratio of the compressor 10 to progressively decrease, and the operating lifetime of the compressor 10 to be substantially reduced.
More specifically, the spring 38 has left and right ends 38a and 38b respectively as illustrated in FIG. 2. When the spring 38 is extended, it generates a lateral reaction force which is directed out of the end 38a as indicated by an arrow 50 in FIG. 3. When the spring 38 is compressed, it generates a lateral reaction force which is directed into the end 38a, or opposite to the direction of the arrow 50.
The lateral reaction forces at the end 38b of the spring 38 are equal and opposite to those at the end 38a. These alternating forces cause the piston 20 to wear into a barrel shape as described above. The spring 48 generates essentially similar reaction forces which cause wear of the piston 22.
As viewed in FIG. 1, the left end portion of the spring 38 is fixed to the left end of the housing 14 by an annular flange 52, whereas the right end portion of the spring 38 is fixed to the armature 30 by an annular retainer 54. Although not illustrated in detail, the flange 52 is fastened to the left end of the housing 14 by screws or the like, whereas the radially inner surface of the flange 52 is threaded. The left end portion of the spring 38 is screwed into the inner threads of the flange 52 and thereby fixedly attached thereto.
The retainer 54 is fastened to the armature 30 by screws or the like, whereas the radially outer surface of the retainer 54 is threaded. The right end portion of the spring 38 is screwed onto the outer threads of the retainer 54 and thereby fixedly attached thereto. The spring 48 is similarly attached to the housing 14 and armature 40 by a flange 56 and retainer 58 respectively.
The prior art arrangement is disadvantageous in that each spring assembly consists of three components: a spring 38,48, a flange 52,56 and a retainer 54,58. The components must be manufactured separately and assembled, which is expensive and labor intensive. In addition, the springs 38 and 48 rub against the mating surfaces of the flanges 52 and 56 and retainers 54 and 58 respectively as the springs 38 and 48 are extended and compressed, generating noise and vibration during operation of the compressor 10 and causing fretting corrosion and wear of the spring.